The present invention relates to a cordless telephone system and a method of controlling such a cordless telephone system.
FIG. 4 shows a schematic representation of a cordless telephone system (CTS). A CTS consists of a fixed part (FP) and one ore more mobile stations (MS). The fixed part and mobile stations are each transceiver stations that can establish call connections. The fixed part is typically connected to a public switched telephone network (PSTN), an integrated services digital network (ISDN) or similar network. Typically, the fixed part is the control element in the CTS, i.e. the call connections to the one or more mobile stations are managed and controlled by the fixed part.
Although the present invention shall be explained in the context of a system represented in FIG. 4, it should be noted that the term xe2x80x9ccordless telephone systemxe2x80x9d should be understood broadly, and it is equally well possible that control functions are distributed over the fixed part and mobile stations, or only over the mobile stations, or that control functions are handled by an external entity that can communicate with both the fixed part and the mobile stations. Furthermore, the fixed part may be connected to a land-based network, but it can also be connected to a radio network, e.g. a cellular telephone network.
Cordless telephone systems are typically arranged such that a call connection between the fixed part and a mobile station comprises an uplink channel (from the mobile station to the fixed part) and a downlink channel (from the fixed part to the mobile station). Therefore, a call connection is associated with a pair of channels. The nature of the channels depends on the communication scheme being used, and a channel can be defined by a frequency, by a time slot, by a combination of frequency and time slot, etc. During a call connection the information is transported via transmission blocks.
A basic problem encountered with cordless telephone systems is that of signal interference, more specifically of channel interference, with sources outside of the CTS, due to the fact that there is no possibility of performing cell planning as e.g. in a cellular network. Such sources of interference can be other cordless telephone systems, cellular telephone systems, machines, etc. For example with respect to cellular systems, the well known solution to avoiding interference is to operate the cordless telephone systems in frequency bands that are well outside of the frequency bands used by the cellular network. Although this avoids interference between the CTS and the cellular network, this does not avoid interference with other sources. Also, as a possibility of more efficiently exploiting the frequency bands assigned to cellular networks, it would be advantageous if a CTS could operate in the same frequency bands as a cellular network. This would also have the advantage that the CTS hardware could be similar to the hardware for a cellular network, e.g. the same transceiver equipment could be employed in the mobile stations for a CTS and a cellular network, and the differences could be implemented in the form of software. This would be a very cost efficient solution.
It is in any case necessary to implement a mechanism in cordless telephone systems that allows a switching from one channel pair to another channel pair, if the transmission quality in the momentary channel pair (in the uplink channel and/or the downlink channel) becomes insufficient. Such an operation will be referred to as a switching procedure in the following.
Although the term xe2x80x9chandoverxe2x80x9d is sometimes used for such a switching procedure, it should be noted that such a handover in a cordless telephone system is completely different from any existing handover in a cellular network, e.g. an intercell handover. More specifically, the intercell handover means that the communication between a given base station and a given mobile station is handed over to a different base station, whereas a call connection between the fixed part and a mobile station remains between these two entities in the case of a CTS switching procedure. Taking GSM as an example, the steps leading to an intercell handover consist in letting a mobile station measure the field strength of the control channels (BCCH) of up to 16 neighbouring base station cells, whereupon the 6 best (strongest field strength) are determined and communicated to the base station with which the mobile station is momentarily associated. The base station controller then processes these 6 indications, and uses the information if the channel to the mobile station deteriorates to a point where a handover to another cell becomes advantageous.
In the case of a cordless telephone system (in which there is no cell planning, such that there is no knowledge of the channels being used by neighbouring CTS systems), the problem consists in finding a suitable new channel pair, once the momentary channel pair deteriorates in quality. The known solution consists in performing measurements on channel pairs that are available or accessible to the system, determining a new channel pair that has desirable qualities, and then switching the call connection from the momentary channel pair to the new channel pair. The problem with this approach consists in the possibility that the steps of measuring and determining a new channel pair can take very long if a complete search is performed during the handover phase or the information on which a channel selection is based is not reliable enough if measurements are performed continuously before a handover condition (such as signal deterioration) occurs, such that a noticeable gap appears in the call connection between the fixed part and the mobile station.
The object of the present invention is to provide an improved cordless telephone system and an improved method of controlling such a cordless telephone system.
This object is achieved by a cordless telephone system having the features of claim 1, and by a method having the features of claim 22. Advantageous embodiments are described in the dependent claims.
An important aspect of the present invention consists in the fact that the interference strength associated with channel pairs other than the channel pair carrying a momentary call connection is measured irrespective of a handover condition being met, such that a pre-selection of candidate channel pairs for a possible handover is determined and collected in a group. Then, if a predetermined condition is met that initiates a switching to a new channel, then this switching or handover to one of the channel pairs of the group is performed very fast, preferably faster than 50 ms. The group may consist of only one channel pair, or may consist of a plurality of channel pairs. If the group consists of more than one channel pair, then further quick measurements can be made to determine the best channel pair, to which the call connection is subsequently switched. In any case, the handover occurs faster than in the prior art, such that no unwanted gaps appear in the call connection.
In the context of the present application, the term call connection should not be understood as being restricted to voice communication, but shall much rather relate to any type of connection, irrespective of the type of information being transmitted, i.e. also encompasses data connections, signalling connection etc.
According to one general aspect of the present invention, the condition initiating a switching procedure is a deterioration of signal quality in the momentary channel pair (uplink channel and/or downlink channel), such that a switching to one of the members of the group candidates is only performed when the quality of the channel pair carrying the momentary call connection becomes unacceptable. According to a another general aspect of the present invention, the condition for switching to a new channel pair occurs regularly, e.g. after a predetermined number of transmission blocks, such that a continuous channel-hopping takes place. In the second aspect, the occurrence of channel deterioration is avoided by continuously hopping to the best channel pair at that moment. The best channel pair is equal to the above-mentioned group, such that in this case the group only has one member.